We have been proposed the new method to measure a dimeter of a micro-sphere on a basis of whispering gallery mode resonance. Resonant wavelengths and mode numbers are necessary quantities to calculate the diameter. In this paper, we proposed and discussed the method to determine the radial mode number of WGMs. Experiments showed the well agreed diameters from various resonant wavelengths, which implies that the radial mode numbers can be successfully estimated by means of the proposed method.